1. Field of the Invention
The present invention relates to auto-ranging ultrasonic attenuation meters such as may be used for detecting voids, inclusions, cracks or other flaws in, for example, solid propellent rocket motors.
2. Description of the Prior Art
One method which has been employed to measure the attenuation of an ultrasonic signal involves the measurement of the amplitude of the attenuated signal waveform on a cathode ray tube (CRT) display. A test specimen is placed between an ultrasonic transmitter and receiver, and the transmitter energized by electrical pulses of 1 .mu.s duration. The receiver output is fed via an emitter follower, attenuator and tuned amplifiers to an input of an ultrasonic test set which has a CRT display. By adjustment of the attenuator and X and Y axis controls of the CRT, the amplitude of the waveform of the input can be measured and compared with the amplitude of the transmitter output signal. One disadvantage of such a method is that the dynamic measurement range is low. In one particular system the dynamic range was 14 decibels (dB) and was only obtained by a continuous manipulation of the system controls.
In a second known method a similar arrangement of emitter follower, attenuator and tuned amplifiers is used but the results are presented as an array of dots produced by a printer instead of a waveform on a CRT display. The number of dots printed is proportional to the amplitude of the received signal. The printing speed of dot printer, which is typically 120 dots per second, imposes a limit on the pulse repetition frequency (prf) of the ultrasonic signal which may be used.